A lot physically separates the sexes, but all of these differences depend on one single element – the Y chromosome.

Researchers have long held the belief that the X and Y chromosome were once identical, until the Y began to evolve and differentiate from the X in males - although the timing of this change was unknown.

Now scientists from Switzerland claim the first ‘sex genes’ appeared in mammals around 180 million years ago and may have been caused by changes in global temperature.

Researchers have long held the belief the X, pictured left, and Y, pictured right, chromosome were once identical, until the Y evolved and differentiated itself, but the timing of this change was unknown. Now scientists claim ‘sex genes’ appeared 180 million years ago and may have been caused by changes in global temperature

THE Y CHROMOSOME MAY BE MORE THAN JUST A SEX GENE

Researchers from Massachusetts have discovered that despite its dramatic decay, the Y chromosome has preserved a small set of genes over millions of years that have ensured the survival of men.

And most have little, if any, role in sex determination or sperm production.

Instead, they have been discovered to play a vital role in determining the differences between the sexes in terms of health and disease.

The findings, published in the journal Nature,
suggest these Y-linked genes could be contributing to differences in
how susceptible the different sexes are to disease, and how severely it
affects them.

By studying samples from several male tissues - taken predominantly from the testicles of different mammal species - researchers from the University of Lausanne's Swiss Institute of Bioinformatics took the Y chromosome genes from the three major mammal lines.

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These included placentals, such as humans, apes, rodents and elephants; marsupials such as opossums and kangaroos; and monotremes - egg-laying mammals which include the platypus and the echidna.

In total, the researchers worked with samples from 15 different mammals, representing these three lineages, as well as the chicken, which they included for comparison.

Instead of sequencing these Y chromosomes, the scientists compared genetic sequences from male and female tissues.

From this they were able to eliminate all sequences common to both sexes in order to keep only those sequences corresponding to the Y chromosome.

Researchers from the University of Lausanne studied the Y chromosome genes from the three major mammal lines. These included placentals, such as humans and elephants, pictured left; marsupials such as opossums and kangaroos, pictured right; and egg-laying mammals which include the platypus and the echidna

By doing so, they established the largest gene atlas of the male chromosome to date.

The study shows that the same sex-determining gene, named SRY, in placentals and marsupials had formed in the common ancestor of both lineages around 180 million years ago.

Another gene, called AMHY, is responsible for the emergence of Y chromosomes in monotremes and appeared 175 million years ago.

Both genes, which according to researcher Professor Henrik Kaessmann are ‘involved in testicular development ,’ and emerged ‘nearly at the same time but in a totally independent way .’

The scientists believe the trigger may have been linked to other sex chromosomes, or environmental factors such as the temperature.

For example, temperature determines sex in present-day crocodiles and may have played a role 180 million years ago.